gheat is an open source application to create heatmap layers for the Google Maps API.
A number of websites have already used gheat to create some interesting heat maps:
The Anglican Church in North America
The Anglican Church in North America is using gheat on their homepage to show Anglican parishes in the USA. From the heat map it seems that Anglicanism is more of an east coast than west coast thing.
Australian Honeypot Project
The Australian Honeypot Project is using gheat to show the locations of computers that are attacking (spamming) the Australian SensorNET. The heat map seems to identify Germany as a particular hot spot for spammers.
Auto and Bike Accidents in Davis, CA
This map is a heat map of auto and bicycle accidents in one town. A heat map is a great way to show the location of accident hotspots.